基于电渣重熔原位Ti和稀土复合微合金化的模具钢一次碳化物组织调控及电极熔速敏感性研究

The development of high-quality die steel is crucial to improve the level of die and mould manufacturing in China and meet the high-technology demand for the market of the high-grade die and mould. Due to the high alloy and carbon content in die steel, coarse primary carbides are directly precipitated from the liquid phase resulting from solidification segregation during conventional manufacturing process. Moreover, these carbides are difficult to eliminate in the subsequent heat treatment and forging processes, which will seriously reduce the performance of die and mould product. In this study, the AISI H13 die steel was selected, and the electroslag remelting (ESR) in-situ Ti and Rare Earth (RE) composite microalloying was proposed. The formation of primary carbides was clarified based on the ESR in-situ microalloying during the solidification of molten steel. Heterogeneous nucleation of primary carbides and austenite by carbon/nitride and RE inclusions was revealed after adding different microalloying elements. The effect of in-situ Ti, in-situ RE, and in-situ composite Ti and RE microalloying on the nucleation and growth of primary carbides was discussed. After that, the effect of melting rate on the primary carbides during ESR in-situ microalloying process was investigated, and the sensitivity of primary carbides during in-situ Ti and RE composite microalloying to melting rate was revealed. From the results of this study, an effective method to improve the size and distribution of primary carbides can be proposed, which will provide the technical support for the production and microstructure control of the industrial die steel.

高品质模具钢的研发是提高我国模具制造水平，满足高技术含量的高档模具市场需求的关键。由于模具钢中合金和碳含量较高，在常规制造过程中因凝固偏析将从液相直接析出粗大的一次碳化物，这些一次碳化物难以在后续热处理锻造过程中完全消除，保留在模具产品中将严重降低钢材性能。本项目拟以AISI H13钢为研究对象，采用电渣重熔原位Ti和稀土复合微合金化的方法调控钢中的一次碳化物，比较添加不同的微合金元素，揭示Ti的碳/氮化物、稀土夹杂物对钢中一次碳化物及奥氏体的异质形核作用，得到原位Ti、原位稀土和原位复合Ti和稀土微合金化对一次碳化物形核生长的影响机制；在此基础上，研究熔速对原位微合金化过程一次碳化物的影响规律，探讨Ti和稀土复合微合金化作用对熔速的敏感性。经本项目的研究可提出改善一次碳化物尺寸和分布的有效方法，为工业化模具钢生产及组织调控提供理论及技术支撑。

模具作为一种高附加值的技术密集型产品，其发展程度已经成为衡量国家制造业水平的重要指标。模具的质量和寿命除了与其合理的设计、制造和使用有关外，主要受模具材料本身的影响。模具钢是制造模具的关键材料，模具钢品质对于我国国产高端模具的市场占有率至关重要。因合金元素凝固偏析形成的一次碳化物是降低模具钢品质、影响其寿命的主要问题，如何改善一次碳化物形态和分布是提高模具钢品质的核心。基于此，本项目开展了以下研究：1）熔速对H13钢电渣重熔熔池温度场和凝固组织的影响；2）电渣重熔原位Ti微合金化、RE微合金化及Ti和RE复合微合金化对H13钢中一次碳化物析出的影响机理。3）电渣重熔原位复合微合金化H13钢的熔速敏感性。通过研究得到了以下主要结果：1）当熔速从0.8kg/min增加至2kg/min时，金属熔池形貌从浅平状变为深V型，局部凝固时间缩短。随熔速增加，H13电渣锭的二次枝晶臂间距减小，Cr、Mo和V元素偏析加重，MC型一次碳化物数量增加。2）电渣重熔原位Ti微合金化促进了钢中细小等轴晶生成，中心区域枝晶生长趋势减弱。Ti微合金化后钢出现少量块状富Ti、V的MN析出相，大尺寸MC碳化物数量降低。Ti含量提高后，MN相尺寸数量均增大，因此，H13钢中Ti含量应该控制在0.03wt%以内。3）电渣重熔原位RE微合金化能够增加H13钢中等轴晶数量，显著改善枝晶间元素偏析，大尺寸MC碳化物数量减少，M2C型碳化物更易形成共晶态。铸锭中夹杂物数量减少超过50%。4）Ti和RE复合微合金化后，一次碳化物细化效果不显著。复合微合金化对MC型碳化物的效果几乎等同于仅含有0.057wt.%Ti的效果。5）随着熔速增加，复合微合金化电渣锭中心区域合金元素偏析程度加大。熔速改变对MC碳化物尺寸影响不大。本研究的成果可为工业生产上改善模具钢的品质合理调控一次碳化物提供理论指导和基础数据。